1. Field of the Invention
This invention relates to the driving of piles into a sea bed and more particularly it concerns novel methods and apparatus for efficiently driving piles underwater with expansible gas driven hammers.
2. Description of the Prior Art
The present invention is particularly suitable for driving piles into sea beds located at large water depths, e.g. five hundred feet (150 meters) or more. With the advent of large offshore towers used to drill for and extract oil and other minerals from the sea bed at offshore locations, it has become necessary to develop ways to drive piles at such depths to anchor the base of the tower securely in place.
One technique used in the past for driving piles into the sea bed involved the provision of an elongated mandrel which extended from the upper end of the pile, at some location under water, to a location up above the water surface. A suitable derrick or crane would support a hammer on top of the mandrel and the hammer would drive the mandrel, and the pile, downwardly into the sea bed. This technique was not suitable for large water depths because the required mandrel length was too great to be handled effectively with available equipment. Also the ability of the mandrel to transmit the hammer blows down to the pile diminished as the mandrel length increased.
Hydraulically driven hammers have been developed which can be operated under water and directly on the pile itself thus eliminating the need for a mandrel. These hydraulic hammers however require complex auxiliary equipment due to the fact that their working fluid is maintained in a closed system. These hydraulic hammers also required special seals and great care had to be taken to insure against leakage of the hydraulic fluid.
Compressible fluid driven hammers have also been developed for underwater operations, one example being shown in U.S. Pat. No. 4,060,139. These compressible fluid hammers are driven by means of a compressed gas, such as air; and they can exhaust into the surrounding sea without contamination. Because of this, no recirculation circuit or special fluid seals are required in connection with these compressible fluid hammers. While compressible fluid driven hammers have provided a considerable improvement over hydraulically driven hammers in underwater pile driving operations, it has been found that when these hammers are used at large depths, i.e. in excess of three hundred feet (90 meters), their efficiency diminishes appreciably.
In copending U.S. applications Ser. No. 743,227 filed Nov. 19, 1976 now U.S. Pat. No. 4,154,307, issued May 15, 1979, and Ser. No. 743,327 filed Nov. 19, 1976, now U.S. Pat. No. 4,138,199, issued Feb. 6, 1979, there are described novel arrangements for utilizing underwater hammers to drive piles which anchor an offshore tower or jacket structure to the sea bed. According to those applications, a plurality of spaced apart guide sleeves are mounted along the legs of the tower or jacket in alignment with each other and these guide sleeves serve to guide the anchoring piles down to precise locations on the sea bed. In addition, the underwater hammer is provided inside an elongated housing which in turn is fed down through and is guided by the guide sleeves onto the top of the pile. The guide sleeves thus not only guide and support the piles for accurate driving, but they also, in combination with the elongated housing, position the hammer on top of the pile.